A key factor in the efficiency of a turbine such as, for example, a heavy-duty gas turbine is the turbine clearance between the blade tips and the casing of the turbine. If the turbine clearance is maintained at a minimum level, the turbine will operate more efficiently because a minimum amount of air/exhaust gas will escape between the blade tips and the casing. Accordingly, a greater percentage of the air and gas entering the turbine will be used to drive the turbine blades and create work.
Due to the different thermal and mechanical growth characteristics of turbine rotor assemblies and the turbine casing, the turbine clearance may significantly change as the turbine transitions between different stages of operation such as from initial start-up to a base load steady-state condition. A clearance control system may be implemented in the turbine to address the turbine clearance conditions during the operation of the turbine.
Prior art clearance control systems typically implement a two stage or two mode control logic. The casing of the turbine is heated for all operating conditions other than base load in order to keep the turbine clearance wide open and prevent any contact between the turbine blades and turbine casing. When the turbine is operating at base load, the turbine clearance will typically be decreased by applying cool air to the turbine casing or, in the case of a two shell turbine containing both an outer and inner turbine shell, by circulating cool air through the inner turbine shell.
These prior art clearance control systems only implement two settings for turbine clearance control, rather than providing continuously modulating clearance control throughout all stages of operation of the turbine. As such, the prior art systems do not make appropriate corrections to the turbine clearance when there are variations to the load of the gas turbine and/or to the ambient conditions in which the gas turbine is operating.
The prior art clearance control systems also typically control turbine clearances according to a preset schedule in which cooling air with a specific flow rate and temperature is utilized to cool the turbine casing. For example, when a turbine is first started, the clearance control system may keep the turbine clearance wide open for a predetermined period of time sufficient for the turbine to reach a base load condition and then begin cooling the turbine casing by circulating cooling air of a predetermined temperature through the turbine casing at a predetermined flow rate. Accordingly, the prior art clearance control systems are unable to constantly monitor and adapt to any changes in the turbine clearance.
Therefore, there exists a need in the art for an improved system and method for monitoring and controlling the turbine clearance of a gas turbine.